A mechanical transmission for a vehicle such as a heavy duty truck will often have two speed ranges, with a number of gear ratios in each range. For example, an eight speed transmission may have a high speed range with four gear ratios and a low speed range with four gear ratios. The transmission will basically be a four speed transmission in combination with an auxiliary or speed range transmission which selects either the high speed range or the low speed range. The speed range is selected by the position of a range selecting rod. The range selecting rod moves between one extreme end position where the low speed range is selected and another extreme end position where the high speed range is selected. At an intermediate position of the range selecting rod, the transmission will be in neutral.
Range selection of the auxiliary or speed range transmission is often operated by pressurized fluid, such as a compressed air system on a truck. In many trucks, for example, the transmission also will have provisions for a power take-off mechanism for driving equipment other than the vehicle wheels. For safety reasons, it may be desirable to prevent engagement of the forward and reverse vehicle drive gears while the power take-off mechanism is engaged. If the auxiliary transmission is accidentally shifted into gear by an operator during operation of the power take-off mechanism while the vehicle is stationary, a catastrophe may occur. U.S. Pat. No. 4,531,422 to Yarnell discloses a transmission shift rod interlock system which includes a spring biased plunger. Before engaging the power take-off mechanism, air pressure is removed from a cylinder/piston device which positions a range selecting rod for normally selecting either the high speed range or the low speed range. When air pressure is removed, the plunger acts on cam surfaces on the range selecting rod to center the range selecting rod in neutral. The operator then engages the power take-off mechanism.